1. Field of the Invention
This invention relates to the field of electronic security, and, in particular, to the encryption and decryption of copy-protected content material.
2. Description of the Related Art
Digital recording techniques are commonly used to record copy-protected content material, such as, audio and video recordings. Subsequent digital copies of such digital recordings are virtually indistinguishable from the original, and offer the same quality as the original.
As the ease of illicitly providing high-quality reproductions of copy-protected content material increases, the need for preventing such reproductions increases. At the same time, however, a legitimate purchaser of such copy-protected content material expects to be able to make copies of the content material for his or her own use. A number of “policy groups”, such as the SDMI (Secure Digital Music Initiative), and others, have been formed in an attempt to reach an equitable compromise between the opposing needs of the owners or vendors of the copy-protected material and the purchasers of copies of the copy-protected material. As a result of the actions of these policy groups, a variety of encryption and decryption techniques have been developed, and continue to be developed, to limit the number of times that a copy of copy-protected content material can be made, or to limit the number of times that a copy may be played-back, or to place an expiration time on a copy. Similarly, encryption and decryption techniques have been developed that limit the type of actions than can be applied to the copy. For example, a copy may have a limit to the number of times that it can be copied, independent of the number of times that it can be played-back. A “copy-once, play-always” authorization would allow for an infinite number of play-backs, but only one copy; a “copy-never, play-10” authorization would allow for ten play-backs, and no copies of this copy. For ease of reference, the term “rendering” is used herein to identify either a recording function or a playback function. For example, a recorder renders the material to a recording medium, a CD-player renders the material to an audio system, a DVD-player renders the material to an audio-visual system, and so on. In addition to limiting the number and type of renderings, the device that provides the limited-use copy may also limit the number of limited-use copies of the copy-protected material that are simultaneously available at any given time. That is, for example, if the number of limited-use copies at any given time is limited to ten, the compliant recorder will not provide an eleventh copy until at least one of the first ten copies is “checked-in”, and marked as being expired, if not already so marked.
In a typical embodiment of a limited-use copy, the copy contains a counter or ticket that stores, in a secure manner, an indication of the authorized rights, and a compliant playback device updates the counter with each rendering or each passage of time, as appropriate for the particular authorized right. In the typical embodiment, the device that provides the limited-use copy and the device that renders the material share a cryptographic key or set of keys that are used to prevent the rendering of the material on an illicit device, and to prevent a modification to the authorization parameters. Typically, the content material is encrypted using a symmetric key, and this key is communicated to the rendering device in an encrypted form, using an asymmetric public key that corresponds to a private key that is associated with the rendering device. In this manner, only the intended rendering device is able to decrypt the encrypted content material. This asymmetric public key is also used to encrypt the authorization rights associated with the encrypted content material.
By limiting the number of simultaneously authorized copies, a compliant provider of the copies cannot be efficiently utilized for an illicit mass production. By limiting the number of times that a copy of copy-protected material can be rendered, the resale value of such a limited-use copy is substantially reduced, thereby diminishing the incentive to illicitly reproduce and sell these copies. At the same time, the purchaser of the original copy of the content material is provided virtually unlimited reproduction rights. The burden on the user of re-recording the expired copies from the original purchased copy is viewed to be minimal, particularly if the number of times that the copy can be rendered before expiring is reasonably high, the number of simultaneous copies is reasonably high, and the effort required for the re-recording is low. The use of an expiration time, in lieu of an expiration based on the number of renderings, can also be used to minimize the resale value of each copy, yet allow the purchaser substantially unlimited reproduction rights. A time-based system based on real time (clock time), however, is not often effective for copy protection, because many illicit copies could be made in a relatively short amount of time, and, conversely, most purchasers would be dissatisfied with a time limit that was not related to whether the material was being rendered during that time. Typically, time-based systems are based on a duration of time that the material is actually rendered, rather than real (clock) time.
Another scenario for the use of time-limited or usage-limited copies of copy-protected material is for the legitimate vendors of the copy-protected material to sell time-limited or usage-limited copies directly, potentially at a lower cost than the above referenced copies that allow for unlimited reproductions. For example, a limited-use copy may be provided via a download from the Internet, or via a broadcast from a provider, such as a cable or satellite television program provider, with an option to purchase an unlimited-use copy. Or, limited-use copies can be provided as rental items, such as a single-use rental of a video recording that does not require the return of the recording within a limited time period. These and other scenarios for the use of limited-use copies of copy-protected content material can be expected to become increasingly common.
One known method of overcoming a limited-use copy protection scheme is termed a “replay attack”. In this method, a bit-for-bit copy is made of the limited-use copy while it contains its full allocation of authorized usage or time, and stored in an archive. Although this copy cannot typically be used in a non-compliant rendering device, because the material is stored in a secured form, this copy can be used, or replayed, on a compliant device by re-recording the bit-by-bit copy of the maximum allocated copy back onto the recording medium. Thus, even though the resale value of a limited-use copy of content material will be lower than the value of an unlimited-use copy, a counterfeiter may choose to provide such illicit limited-use copies, because of the ease of creating the copies, and the ease of overcoming the limited-use copy protection scheme.